Nick Clegg did something really important this week. Its significance has not yet been fully recognised. By ruling out the possibility of the Liberal Democrats forming a coalition with Labour or the Conservatives after the next election Clegg did not just pre-empt the endless campaign trail questions about who he is more likely to shack up with. He also blew away a lot of the wishy-washy talk surrounding the whole subject of hung parliaments.

None of this is to say a hung parliament is either unlikely or unimportant. Quite the reverse. But it means that the debate about it ought to get real. The reality is that there will not be a coalition in a hung parliament. There will be a minority government. What is more, it is overwhelmingly likely to be a Conservative minority government. Since that is the clear implication of Clegg's approach I conclude it is also an outcome he is prepared to face. So must we all.

This week it was reported that the cabinet secretary Gus O'Donnell has been circulating colleagues with memos drawn up by his 1970s predecessors during the last period that Britain had a hung parliament. How far these memos might apply in 2010 is anyone's guess, but they are certainly a better guide to the realities than some of the more fanciful current speculations.

Robert Armstrong's "Note for the record" on the fall of Edward Heath's government in 1974 was written by Heath's principal private secretary less than a fortnight after the events took place. It describes at length, and sometimes in poetic detail, how Heath fought for three days to defy his general election reverse and remain in office. If Armstrong's memo doesn't give us a pretty accurate insight into how Gordon Brown would try to cling to power in similar circumstances, I'm a Dutchman.

At all times, Armstrong's note stresses the decisive importance of the parliamentary arithmetic. If the Tories were the largest party but with no overall majority, Heath told Armstrong, "his inclination would be to stay in office and meet parliament, rather than make any approach to the Liberals." There was precedent for that "defeated but still largest party" approach from the hung parliaments of 1886, 1892 and 1923, though in each case they were voted down. Nevertheless, everything we know about Brown suggests that he would take Heath's view, too.

If Labour is the largest party after the 2010 election, I confidently predict that Brown will fight on alone. It is in his party's DNA to do this. To have emerged as largest party would seem almost a victory to many Labour MPs. The pressure not to hand the keys of No 10 to David Cameron would also be intense. And it is certainly in Brown's own DNA to hold on to power. He would want to talk to Clegg about Lib Dem intentions, but I doubt a coalition would be seriously offered and none, we now know, would be accepted.

End of story? Hardly. The key question would then become whether Brown would win a Queen's speech vote. If Labour is a couple of seats shy of a majority its chances of survival would be higher than if they are 20 short. But the Lib Dems would come under immense pressure to vote against Brown either way. With a second election highly possible, the charge of propping up a defeated Labour party could be self-inflicted electoral disaster. In the end, Clegg would surely do what Asquith did in 1924 and swing his party against the government's effort to cling on.

In 1974, though, Heath's position was even weaker, as Brown's may also be. Labour emerged from the election with a handful more MPs than the Tories. Yet Heath still tried to hold on, offering Jeremy Thorpe a seat in the cabinet, the possibility of other Liberal ministers and a Speaker's conference on electoral reform, in an effort to keep Labour out. Brown would doubtless try something of the sort in an effort to keep Cameron from power. But it did not work in 1974 and it would not work in 2010. Thorpe was up for it, but his party would not buy. Nor would Clegg's. No deal.

Armstrong's memo contains many other reminders of how history may again, in Mark Twain's word, rhyme with the present. The Ulster Unionists were straight on the phone to Heath, offering the poisoned chalice of their support. So were wannabe freelance powerbrokers like Woodrow Wyatt and the megalomaniac media magnate Cecil King's wife Ruth. Teddy Taylor offered to be go-between to the Scottish Nationalists. Brown can expect plenty of calls from Belfast and the self-important. He will make a lot of calls too. The bunker will go into overdrive. Thorpe called 1974 a nightmare on stilts for Heath. This time it could be even worse.

There is much in Armstrong's memo about what the Queen should do. In the end, rightly, Armstrong concluded that she could only await events. It would be the same in 2010, only more so. The idea canvassed by Armstrong and Lord Crowther-Hunt in 1974 that the Queen might consult some national elders – Harold Macmillan and Manny Shinwell were suggested – and might even invite Willie Whitelaw or Roy Jenkins to form a government was fanciful even then.

Today such a "Miliband option", as it is optimistically dubbed, would be even more for the birds. If Brown was persuaded to step down in order to enable Labour to govern, there is no way the palace could invite any Labour politician except Harriet Harman to form a government. In earlier times, private soundings might have produced consensus prime ministers like Churchill or Douglas-Home. Today parties are encumbered with more rules. If Brown goes, Harman automatically becomes Labour leader until a party leadership election is held. The Miliband option is a non-starter.

History cannot illuminate the future. It is, in Coleridge's lovely image, a lantern on the stern of the ship, illuminating the waters through which we have passed. Where hung parliaments are concerned, though, it tells us that defeated governments cannot snatch victory from the jaws of defeat. A hung parliament therefore means that there will be a minority Conservative government, nothing more and nothing less.

After three days trying to postpone the inevitable, Armstrong went with Heath to the palace. "On the drive neither of us said a word. There was so much, or nothing left, to say," he wrote. At the palace, Armstrong almost burst into tears. Will this all be re-enacted by Brown and his own private secretary? Like so much else in British politics right now, that all depends on the voters. But Clegg has made it a little more likely.